The aforesaid recording techniques are considered to be particularly interesting from the viewpoint of achieving improvements in storage density with respect to current storage techniques, substantially based on a “longitudinal” utilization of the recording media.
As opposed to “longitudinal” recording techniques, the waveform read in “vertical” recording systems exhibits a significant energy level even at very low frequencies.
Since, for various reasons, the stages of the read chain are mutually AC-coupled, a certain loss of energy is entailed, which is more noticeable than that occurring in “longitudinal” magnetic recording systems.
To limit this drawback, the AC-coupling frequency is normally kept below 1% of the sampling frequency.
Nevertheless, the system is expected to be capable of tracking constant non-zero levels over an indefinite period of time; this is clearly impossible, since even a low AC-coupling gives rise to an appreciable reaction after more than a hundred samples: this phenomenon is known in the art as the Base Line Wander, or BLW for short.
Dynamic compensation of this phenomenon utilizing a feedback loop has been proposed. The corresponding reaction time is limited, however, due to both the latencies intrinsic in the implementation and the need to realize a decoupling from the control loops, such as those that perform control of the gain, equalisation and sample timing functions.
Shorter term phenomenon, known as “droop”, also arises in the same context due to the formation of a polarization charge on the AC-filter after a series of constant non-zero levels. This effect can be successfully compensated by limiting the maximum time between two consecutive signal transitions, a well known technique known as RLL, an acronym for Run Length Limited, encoding.
At the present time, there are known read/write systems for vertical magnetic recording that use a 20/21 RLL encoding for counteracting the droop effect and a combination of feedback and DC-coupling optimisation for minimizing the BLW effect.